An optical density value greater than or equal to 0.15 at A650 n m of amniotic fluid obtained through transabdominal amniocentesis from a high risk obstetrical population, which excluded diabetes mellitus, isoimmunization, and amniotic fluid infection, correlated 98.6% with a mature lecithin to sphingomyelin ratio (L/S) greater than or equal to 2.0% and 41.6% with the presence of phosphatidylglycerol (PG). The corresponding false positive rates were 1.4% and 16.8%, respectively. In 30 infants delivered within 48 h of testing, A650 n m greater than or equal to 0.15 correlated 100% with absence of respiratory distress syndrome. A650 n m is not recommended for Rhesus immunized gestations, diabetic pregnancies and amniotic fluid infection due to a high false positive rate with A650 nm compared to L/S and PG.